Causes of groundwater salinization in a low lying area of Cochabamba Valley, Bolivia

The recent salinization of shallow groundwater in a depression in Cohambamba Valley, Bolivia, was examined by using geochemical and isotopic techniques. Groundwater C1-/Br- ratios in centre of this depression suggest that recharge of highly evaporated water occurred in the past although groundwater has subsequently been diluted. Depleted δ18O and δ2H values and low deuterium excesses (d = 5.1-5.8) in the centre of the basin indicate that groundwater was recharged during a cooler climate. The absence of 3H and low 14C concentrations (down to 11 pmc) in groundwater also suggests older groundwater ages. Thus the dilution observed could have been caused by recharge during the more humid, cool climate of a glacial epoch. Relative dating of 14C data, although not conclusive, given evidence of methanogenesis occurring in the centre of the basin, suggests a groundwater age of approximately 12,000 years, which corresponds to a glacial period. Na+/Ca++ ratios are much higher in the basin than in the major aquifer systems along the north side of the valley. This difference could be due to the dominance of clay aquitards in the depression, where both saline residual pore waters and ion exchange could cause groundwater to become increasing sodium rich. Higher silica and fluoride concentrations suggest longer residence times for groundwater in the basin. Furthermore, an abnormally high groundwater geothermal gradient exists in this area, suggesting slower groundwater velocities. Thus, groundwater salinization is due, at least in part, to the presence of saline groundwater at depth and to the use of this water for irrigation, although cultivation and irrigation practices may aggravate the salinity problem. (author). 21 refs, 4 figs, 1 tab